function limitDrag(id){			//不要写成Function，又不是构造函数
	Drag.call(this, id);

	for (var i in Drag.prototype){
		limitDrag.prototype[i] = Drag.prototype[i];
	}

	limitDrag.prototype.fnMove = function (ev){
		var ev = ev || event;
		var L = ev.clientX - this.disX;
		var T = ev.clientY - this.disY;

		if (L > document.documentElement.clientWidth - this.oDiv.offsetWidth) {
			L = document.documentElement.clientWidth - this.oDiv.offsetWidth
		} else if (L < 0) {
			L = 0;
		};

		if (T > document.documentElement.clientHeight - this.oDiv.offsetHeight) {
			T = document.documentElement.clientHeight - this.oDiv.offsetHeight;
		} else if (T < 0) {
			T = 0;
		};

		this.oDiv.style.left = L + 'px';
		this.oDiv.style.top = T + 'px';
	}
}